1. Field of the Invention
This invention relates to oil film bearings of the type employed to rotatably support the necks of rolls in rolling mills.
2. Background Discussion
A rolling mill oil film bearing typically comprises a sleeve axially received on and rotatably fixed to a roll neck. The sleeve is surrounded by a bushing contained in a chock mounted in a roll housing. In service, the sleeve is rotatably supported on a thin film of oil hydrodynamically maintained at the bearing load zone between the sleeve and the bushing.
The sleeves may be either internally cylindrical for use on cylindrical or xe2x80x9cstraightxe2x80x9d roll necks, or they may be internally tapered for use on tapered roll necks. Moreover, the sleeves may be xe2x80x9cself lockingxe2x80x9d or xe2x80x9cnon-lockingxe2x80x9d. Self locking sleeves are frictionally fixed to the roll necks by interference fits, whereas non-locking sleeves require keys or the like to effect a mechanical interengagement with the roll necks.
The present invention is concerned with an improvement to internally tapered sleeves having relatively large journal diameters of at least 500 mm and non-locking taper angles of at least three degrees.
With reference to FIG. 1, one such sleeve is shown at 10 as a component part of a rolling mill oil film bearing 12. The sleeve 10 is removably received on a tapered section 16 of the roll neck and is rotatably fixed to the roll neck by keys 14 or the like. The sleeve is surrounded by a bushing 18 and fixed in a bearing chock 20. In service, as previously noted, the sleeve is rotatably supported on a thin film of oil (not shown) hydrodynamically maintained at the bearing load zone between the sleeve and the bushing.
With reference to FIG. 2, it will be seen that the sleeve 10 has an internally tapered section 21 with a length L, an end section 22 extending axially beyond the outboard end of the internally tapered section, and a cylindrical outer surface 23 surrounding the internally tapered section. The cylindrical outer surface has a diameter D (also commonly referred to as the xe2x80x9cjournalxe2x80x9d diameter). The internally tapered section has a taper angle xcex1, a minimum thickness t at its inboard end, and a maximum thickness txe2x80x2 at its outboard end adjacent to the end section 22. Keyways 15 coact in mechanical interengagement with the keys 14, with both the keys and the keyways being located outside of the internally tapered section 21 and exclusively within the end section 22.
The journal diameter D and length L of the internally tapered section 21 govern the size and load bearing capacity of the bearing. The taper angle xcex1 will dictate whether the sleeve is self locking or non-locking.
When this type of oil film bearing was first introduced to rolling mills in the 1930""s, those skilled in the art deemed it essential that the sleeve wall at the load zone have a minimum thickness t sufficient to withstand elastic deformation of the sleeve under load conditions, and also to accommodate a maximum thickness txe2x80x2 sufficient to resist damage at the keyways due to torque induced stresses. Thus, as depicted by the plot line 24 in FIG. 5, for sleeves having journal diameters ranging from 500-2100 mm, minimum thicknesses t typically ranged from 30 to 70 mm, with the average being t=0.024 D+22.6 as represented by the broken plot line.
This design criteria remained virtually unchanged until the mid 1970""s, when, as disclosed in U.S. Pat. No. 4,093,321, those skilled in the art identified the interruption of the internally tapered section by the keyways as a cause of dimensional irregularities in the products being rolled. In order to correct this problem, the keys and keyways were moved out of the internally tapered section 21 to their present positions located exclusively in the end section 22, as shown in FIGS. 1 and 2. Although this obviated any necessity to maintain an increased thickness txe2x80x2 at the outboard end of the internally tapered section, sleeve thicknesses remained unchanged due to the continuing belief on the part of those skilled in the art that load induced elastic sleeve deformation was detrimental and to be avoided at all costs.
It has now been discovered, however, that contrary to conventional wisdom, a certain amount of elastic sleeve deformation is not only tolerable, but desirable in that it results in an increase in the surface area of the sleeve supported on the oil film at the bearing load zone. This in turn increases the load bearing capacity of the bearing. Thus, for a given journal diameter D and taper angle xcex1, a beneficial magnitude of elastic deformation can be introduced by reducing the minimum thickness t, which for a given length L and taper angle xcex1, results in a reduction in thickness of the sleeve throughout the length of the internally tapered section. Preferably, the reduction in thickness t is achieved by increasing the bore size of the sleeve while maintaining the journal diameter D constant. This enables the sleeve to accommodate a larger roll neck, thereby further strengthening the overall assembly.
A principal objective of the present invention is to increase the load bearing capacity of relatively large oil film bearings having internally tapered non-locking sleeves by at least 10%, and preferably by as much as 20% or more through appropriate reductions in the minimum sleeve thicknesses t.
A companion objective of the present invention is to reduce the size and weight of the metal forgings or castings from which the sleeves are machined, thereby beneficially reducing raw material costs.
These and other objectives, advantages and features of the present invention will now be described in greater detail with continued reference to the accompanying drawings, wherein: